The vitamin D resistant disorders include a varied group of bone diseases refractory to treatment. Recently we have established that X- linked hypophosphatemic rickets (XLH), the prototypic such disorder, is due to the extrarenal production of an hormone or metabolic factor that inhibits Na+-phosphate cotransport, resulting in phosphate (Pi) wasting. Moreover, we have successfully cloned the gene (PEX) responsible for this disease. These accomplishments provide the basis for studies that will improve our understanding of Pi homeostasis and the abnormalities underlying XLH. In initial investigations we will extend our knowledge regarding the PEX gene. Using SSCP technology and direct sequencing, we will identify and localize point mutations in patients with XLH in order to define the crucial epitopes of the PEX protein. In addition, we will use cDNA from the PEX gene to isolate the murine counterpart which will permit us to study in depth its tissue expression and physiologic regulation under a variety of conditions. As a complement to these studies, we will conduct experiments designed to identify and characterize the humoral factor upon which the PEX protein likely acts and which effects abnormal Na+-Pi cotransport in the kidney. We will assess the biological characteristics of the Pi transport inhibitory activity present in the conditioned medium from immortalized hyp-mouse hepatocytes (or osteoblasts). In addition, we will make detailed efforts to assure that the factor produced in vitro acts as phosphatonin in vivo. Moreover, we will use standard techniques to isolate the protein factor and perform structural analysis. Finally, we will assess what elements of the hyp-mouse phenotype are phosphate independent, but PEX dependent. These studies will include comparison of various phenotypic characteristics in hyp-mice with those in NPT-2 gene knockout mice. Collectively, these investigations will provide new data regarding Pi homeostasis, as well as the pathogenesis of XLH and related vitamin D resistant diseases. Such advances will potentially improve the therapeutic strategies for a variety of human disorders. Indeed, we hope to apply this information to develop and test such new treatments for XLH and other related diseases.